A study on the physico-chemical structure of activated sludge flocs was carried out to get a better insight in its relation with sludge settleability. For this purpose, sixteen sludges from different origins were analysed in order to provide information with regard to their settleability, biomass and exocellular composition, surface characteristics and internal hydrophobicity. The presence of filamentous micro-organisms was observed in all samples but was not always associated with poor settleability, supporting to some extent the idea of their role as a backbone in the flocs. The relations between variables were studied through their linear correlations. Exocellular polymers (ECP) were associated with poor settling conditions. The chemical composition of the ECP (the DNA fraction and the C/N ratio) had an influence on the surface characteristics of the flocs. Finally, sludge settleability was described with a mathematical model, opposing the ECP and the internal hydrophobicity of the flocs. The positive role of hydrophobic interactions, in term of settleability, should provide a new approach in the understanding of flocculation mechanisms in activated sludge.

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